82 Rb Generating method and eluent

ABSTRACT

The present invention provides a low  82  Sr breakthrough eluent and method of generating  82  Rb from  82  Sr from a  82  Sr charged inorganic adsorbant column. Eluting is done with a pharmaceutically acceptable saline and buffer solution, which is preferably isotonic  82  Sr breakthroughs of 10 -8  are obtained at clinically useful elution rates greater than 10 ml per minute. Phosphate and carbonate buffers are preferred. Al 2  O 3  and ZrO 2  are preferred inorganic radiation damage resistant adsorbants.

BACKGROUND OF THE INVENTION

Rubidium --82, a positron emitter with a half-life of 75-sec is readily obtainable from the parent Sr-82 (T_(1/2=) 25 days). Rubidium can be used as a diffusible flow tracer for the myocardium and kidney, and as a nondiffusible tracer for brain blood flow. Serial injections of Rb-82 can be administered every 5 to 10 minutes by eluting (milking) Rb-82 from its 25-day Sr-82 parent. The advantages of Rb-82 are low radiation dose, ability to provide for repeated examinations every 5 minutes without constraints from body background, and a convenient and economical supply of a short-half-life positron emitter. (Yano et al, The Journal of Nuclear Medicine 20:961-966, 1979.)

Significant quantities of ⁸² Sr are available for clinical investigation. The short-lived daughter, 75-second ⁸² Rb, is of value in biomedicine for circulation and perfusion studies as well as for myocardial imaging as mentioned in U.S. Pat. No. 3,953,567.

Grant et al disclose partial resolution of inconsistencies in the medical literature regarding the performance characteristics of ⁸² Sr/⁸² Rb radionuclide generators as discussed at page 1250 of Grant el al, The Journal of Nuclear Medicine, Vol. 19, Number 11; pages 1250-1254, 1978.

Yano et al, Journal of Nuclear Medicine, 18:46-50, 1977 disclose that two different ion-exchange resins loaded with spallation-produced ⁸² Sr indicated that the Bio-Rex 70 Saline system was superior to the Chelex-100 NH₄ Cl--NH₄ OH system for the separation of ⁸² Rb. Not only was the observed separation factor higher with Bio-Rex 70 resin, but a 2% saline solutin was also observed to be a better eluent for intravenous infusion than the 0.1 M NH₄ OH--NH₄ Cl buffer.

Yano et al in the International Journal of Applied Radiation & Isotopes, Vol. 30, pages 382-385, 1979, disclose breakthrough data for ⁸⁵ Sr batch studies with Al₂ O₃ adsorber using 2% NaCl, pH 8-9 eluent; Bio-Rex 70 adsorber using 2% NaCl, pH 8-9 eluent; and Chelex 100 adsorber using 0.1 M (NH₄ Cl+NH₄ OH), pH 9 eluent. Yano et al cite previous work with Bio-Rex 70, a weakly acidic cation resin and Chelex 100, a chelating ion exchange resin, which indicated that a good separation of ⁸² Rb from ⁸² Sr could be obrained; however, the former ion exchange resin exhibits an increase in ⁸² Sr breakthrough after a moderate number of elutions with 2% NaCl at pH 7-8, while the latter resin requires an NH₄ OH+NH₄ Cl buffer at pH 9.0 as the eluent solution which is not desirable for intravenous infusion.

Rubidium and potassium are chemically related elements and are in the alkali-metals group of the periodic table. The biological behavior of Rb and K is very similar, both being taken up by muscle. Furthermore the myocardial uptake of Rb after intravenous infusion is related to the rate of blood flow through the myocardium. ⁸⁴ Rb, a positron-emitting isotope, has been used with coincidence gamma-ray counters to determine coronary blood flow in man. However, because of its relatively long half-life of 33 days and its high cost, ⁸⁴ Rb presents disadvantages for coronary blood-flow studies. ⁸² Rb has physical characteristics that are suitable for visualizing deep-lying organs. It has a half-life of 75 sec and decays 96% of the time by positron emission with a maximum energy of 3.15 Mev. The positron is accompanied by a 0.77-Mev gamma ray (9.0% abundant) to the ground state of ⁸² Kr. Its very short half-life offers low radiation exposure and the possibility of quick repeat studies. Because it is a positron emitter, the positron scintillatin camera, with its high sensitivity and excellent image-forming characteristics for deep-lying organs can be used; see Yano et al, Journal of Nuclear Medicine, volume 9, Number 7 pages 412-415; 1968.

⁸² Rb is produced continuously by decay of the parent isotope, ⁸² Sr which decays with a half-life of 25 days. By use of a chromatographic column, ⁸² Rb can be milked from the parent isotope every 5-10 , minutes.

SUMMARY OF THE INVENTION

The present invention provides a low ⁸² Sr breakthrough eluent and method of separating ⁸² Rb from a ⁸² Sr charged inorganic adsorbant column. Eluting is done with a pharmaceutically acceptable saline and buffer solution, which is preferably isotonic. ⁸² Sr breakthroughs of 10⁻⁸ /ml are obtained at clinically useful elution rates greater than 10 ml. per minute. Phosphate and carbonate buffers are preferred. Al₂ O₃ and ZrO₂ are preferred inorganic radiation damage resistant adsorbants.

DETAILED DESCRIPTION OF THE INVENTION

The method and eluents of the present invention are useful in position imaging and quantitation of blood flow through the myocardium, brain and kidneys.

The present invention provides improved breakthrough characteristics. It has been discovered that breakthrough of Sr may be lowered by providing a pharmaceutically acceptable buffer in a pharmaceutically acceptable saline eluent.

an inorganic ion exchange adsorbant column is used because of good resistance to radiation damage. The eluents of the present invention are suitable for intravenous infusion.

Preferably the buffer is a phosphate salt or a carbonate salt. Most preferably the buffer is a phosphate salt.

Bacteriostats may be beneficially added to the eluent. Preferred bacteriostats are those which are pharmaceutically acceptable buffers, for example parabens.

The eluent may be buffered at a pharmaceutically acceptable pH. Preferably the pH is from 6.0 to pH 10. Most preferably the pH is from pH 7.5 to pH 9.5. the concentration of the buffer in the eluent preferably is from .01 mmol to 200 mmol per liter of eluent solution.

The saline concentration of the eluent is a pharmaceutically acceptable concentration. Preferably the saline is isotonic (0.9%).

A column containing inorganic adsorbant is charged with ⁸² Sr. Preferably the inorganic adsorbant is Al₂ O₃ or ZrO₂. the column is then eluted with the eluent. Preferably the column is eluted at greater than 10 ml per minute.

At clinically useful flow rates of about 50 ml per minute, ⁸² Sr breakthrough of 10⁻⁸ per ml of eluent are obtained by the present invention. Breakthrough is the ratio of microcuries of ⁸² Sr in the eluent to the microcuries of ⁸² Sr in the adsorber.

Phosphate salts include alkali phosphates, alkaline earth phosphates, alkali metal hydrogen phosphates, alkaline earth hydrogen phosphates as well as hydrates of phosphate salts. Also phosphate salts include all phosphorous oxides which form phosphates upon addition to water.

A preferred phosphate salt is Na₂ HPO₄ which may be added to the eluent as Na₂ HPO₄ ·7H₂ O. In the saline eluent it forms Na+ and PO₄ ⁻³ ⃡ HPO₄ ⁻² ⃡ H₂ PO₄.sup.⊖. Upon addition of NaOH some of the H₂ PO₄.sup.⊖ would be taken up in the formation of HPO₄ ⁻². The balanced equation being:

    NaH.sub.2 PO.sub.4 +NaOH⃡Na.sub.2 HPO.sub.4 +H.sub.2 O

When acid is added for example HCl; some H₂ PO₄.sup.⊖ is formed. The balanced equation being:

    Na.sub.2 HPO.sub.4 +HCl⃡NaH.sub.2 PO.sub.4 +NaCl.

Carbonate salts include water soluble carbonate salts such as alkali metal carbonates and alkali metal hydrogen carbonates for example NaHCO₃. In water NaHCO₃ forms Na⁺ and CO₃ ⁻² ⃡H CO₃.sup.⊖ ⃡H₂ CO₃. Upon addition of NaOH; HCO₃ ⁻¹ and H₂ CO₃ are taken up and CO₃ ⁻² and HCO₃ ⁻¹ respectively are formed. Upon addition of HCl; CO₃ ⁻² and HCO₃ ⁻ are taken up and HCO₃ ⁻ and H₂ CO₃ respectively are formed.

The buffer of the present invention controls the amount of ⁸² Sr breakthrough. Much lower ⁸² Sr breakthrough is obtained where the saline eluent is buffered than where saline alone in aqueous solution is used. Also where the column material is pre-equilibrated with the buffer solution reduced ⁸² Sr breakthrough is obtained.

Examples 1 and 2 are specific embodiments of the invention.

EXAMPLE 1

A standard sized Minitec generator (commercially available from E. R. Squibb and Sons, Inc., Princeton, New Jersey) is used. The column is filled with Al₂ O₃ (2 cc bed volume of Basic Woelm). The column is then pre-equilibrated by washing with 0.025% (dry weight) of Na₂ H PO₄ . 7 H₂ O aqueous isotonic saline solution. The column is loaded with 0.5 cc (500 micrograms) of a mixture of ⁸² Sr, ⁸⁵ Sr and ⁸³ Rb in isotonic saline and sodium hydroxide solution having pH 12. 0.5 cc of air is pulled from the lower end of the column while loading the activity. The column is allowed to set for 2 hours for adsorbtion to take place. Then the column is eluted with 0.025% (dry weight) of Na₂ HPO₄ . 7H₂ O aqueous isotonic saline solution.

EXAMPLE 2

the procedure of Example 1 is used except that autoclaving is carried out in this example.

Table 1 shows the breakthrough fraction per ml of eluent from the column used in Examples 1 and 2 and the pH of the eluate for the volume ranges of eluent shown. the elution speed is 50 ml/min.

                  TABLE I                                                          ______________________________________                                                         Breakthrough fraction of                                                       .sup.82 Sr/ml;                                                 Volume (ml)           Example 1 Example 2                                      ______________________________________                                          0-200   (.sup.83 Rb Elution)                                                                        ≦4 × 10.sup.-8                                                              ≦4 × 10.sup.-8                    200-300  400 ml eluated                                                                 at 50 ml/min.                                                         800-900               ≦5 × 10.sup.-8                                                              ≦1.5 × 10.sup.-8                  1400-1500             ≦10.sup.-8                                                                        ≦10.sup.-8                              2000-2100             ≦2 × 10.sup.-8                                                              ≦3 × 10.sup.-8                             Columns rested                                                                 4 days then 1st                                                                eluate collected                                                      2100-2200                                                                               (1st)         4 × 10.sup.-7                                                                     10.sup.-6                                      2200-2300              5 × 10.sup.-8                                                                     1.4 × 10.sup.-7                          2800-2900             ≦2 × 10.sup.-8                                                              ˜5 × 10.sup.-8                     3400-3500             <3 × 10.sup.-9                                                                     3 × 10.sup.-8                            4000-4100             <3 × 10.sup.-9                                                                     1.5 × 10.sup.-8                          4600-4700             ˜6 × 10.sup.-9                                                               9 × 10.sup.-8                            5200-5300             ˜6 × 10.sup.-9                                                               1.1 × 10.sup.-7                          5800-5900             <3 × 10.sup.-9                                                                     1.3 × 10.sup.-7                          6400-6500                                                                               Blown dry and                                                                               <3 × 10.sup.-9                                                                     1.5 × 10.sup.-7                                   rested (rested                                                                 2 days)                                                               6500-6600                                                                               Column rested 2                                                                              2.5 × 10.sup.-8                                                                   1.5 × 10.sup.-7                                   days                                                                  6600-6700             <5 × 10.sup.-9                                                                     5 × 10.sup.-8                            7200-7300              2 × 10.sup.-8                                                                     6 × 10.sup.-8                            7800-7900             <10.sup.-8                                                                               7 × 10.sup.-8                            7900-8000              5 × 10.sup.-8                                     ______________________________________                                    

The procedure used in the examples is that the generator is eluted for 2 minutes at 50 ml per minute resulting in 100 ml of eluate. The 100 ml fraction is then counted on a Ge(Li) detector for 777 KeV ⁸² Rb gamma ray.

2.2μ Ci of ⁸² Sr is used as a comparative standad from which to calculate the activity of ⁸² Sr in the 100 ml eluate. 

We claim:
 1. A low ⁸² Sr breakthrough method of generating ⁸² Rb from a ⁸² Sr charged inorganic adsorbant column comprising eluting the ⁸² Rb from the said inorganic adsorbant with an eluent comprising a pharmaceutically acceptable saline and buffer solution.
 2. The method of claim 1 wherein said inorganic adsorbant is Al₂ O₃ or ZrO₂.
 3. The method of claim 1 wherein said eluent comprises a phosphate salt or a carbonate salt and isotonic saline.
 4. The method of claim 1 wherein said eluent solution is buffered at pH 6.0 to pH 10, said column is eluted at greater than 10 ml per minute, and said eluent solution is isotonic and acceptable for intravenous infusion.
 5. the method of claim 4 wherein said eluent solution comprises a carbonate salt.
 6. The method of claim 4 wherein said column is pre-equilibrated with a buffered isotonic saline solution.
 7. The method of claim 1 wherein said eluent solution further comprises a bacteriostat.
 8. A pharmaceutically acceptable solution comprising ⁸² Rb, a saline solution and a buffer.
 9. A pharmaceutically acceptable solution in accordance with claim 8 wherein the saline solution is isotonic.
 10. A pharmaceutically acceptable solution in accordance with claim 8 further comprising a bacteriostat.
 11. A pharmaceutically acceptable solution in accordance with claim 10 wherein the buffer and bacteriostat are a single ingredient.
 12. A pharmaceutically acceptable solution in accordance with claim 8 wherein the buffer is a phosphate salt or a carbonate salt. 